The present invention relates to an RF (radio frequency) coil, RF magnetic field generating apparatus and magnetic resonance imaging method and apparatus, and more particularly to an RF coil having a plurality of electric paths carrying electric current for generating an RF magnetic field and an annular electric path linking respective ends of the plurality of electric paths, an RF magnetic field generating apparatus employing such an RF coil, and a magnetic resonance imaging method and apparatus employing such an RF magnetic field generating apparatus.
A magnetic resonance imaging apparatus having the direction of a static magnetic field perpendicular to the body axis of a subject, commonly referred to as a vertical magnetic field-type magnetic resonance imaging apparatus, generates an open static magnetic field space, and employs as an RF coil for generating an RF magnetic field an RF coil having a loop surface parallel to pole piece surfaces of a static magnetic field generating unit. The RF coil of this type generates the RF magnetic field in the direction parallel to the coil loop surface to form the RF magnetic field perpendicular to the direction of the static magnetic field. Such an RF coil is disclosed in U.S. Pat. No. 5,760,583 of the present applicant.
On the other hand, there is another type of an RF coil referred to as a quadrature RF coil, which consists of a combination of two RF coils, and can provide an RF magnetic field having an increased strength by vector composition of respective RF magnetic fields generated by the two RF coils or can reduce a drive power per coil to obtain the RF magnetic field of a desired strength.
However, in the conventional RF coils, an unwanted RF magnetic field is generated from electric current flowing through tie paths for linking main paths. Since the unwanted RF magnetic field cancels the RF excitation magnetic field generated from the main paths, efficiency in generating the RF magnetic field is reduced and a larger electric power for excitation is required to obtain a desired magnetic field strength.
Moreover, the electric current flowing through the main paths and the tie paths becomes larger correspondingly, raising additional problem that the high frequency electric power absorption, i.e., the whole SAR (specific absorption rate), of the subject is increased.
Furthermore, the unwanted RF magnetic field generated by the tie paths not only cancels the RF excitation but also increases the local high frequency electric power absorption, i.e., the local SAL, of the subject.